1. Field of the Invention
The present invention relates to a sheet processing apparatus capable of buffering (storing) a sheet supplied during a processing operation of a sheet processing unit, which processes a sheet, and to an image forming apparatus including the sheet processing apparatus in an apparatus main body thereof.
2. Description of the Related Art
Up to now, an apparatus main body of an image forming apparatus for forming an image on a sheet includes a sheet processing apparatus, which processes a sheet conveyed from the apparatus main body connected thereto or incorporated therein (see JP 2004-269165 A). As examples of the image forming apparatus, there are given an electrophotographic copying machine and a laser beam printer.
A conventional sheet processing apparatus has a structure in which, as shown in FIG. 22A, sheets each having an image formed thereon are sequentially supplied from the apparatus main body of the image forming apparatus, supplied sheets P are stacked on an intermediate tray 29 in a bundle, and then, for example, the sheets are stapled using a stapler (not shown). In this case, a sheet processing apparatus 19 cannot stack the sheets sequentially supplied from the apparatus main body of the image forming apparatus on the intermediate tray 29 while the stapler performs a stapling operation.
Accordingly, the sheet processing apparatus 19 buffers (stores) a predetermined number of sheets P1, P2, and P3 supplied during the stapler operation by a buffering unit 40 as shown in FIG. 22B (see JP 2004-269165 A, JP 2001-220050 A, JP 2004-210534 A, and JP 2004-246056 A). A buffered sheet is referred to as “buffer sheet”, and the number of buffer sheets is three, for example.
The sheet processing apparatus shifts a stapled sheet bundle P to a downstream side by a distance L with respect to the buffer sheets P1, P2, and P3 by a trailing edge assist 34 as shown in FIG. 23A. After that, as shown in FIG. 23B, a rocking roller pair 27 nips the stapled sheet bundle P and the buffer sheets P1, P2, and P3 and conveys at the same time.
Finally, the rocking roller pair 27 rotates to deliver the stapled sheet bundle P onto a stack tray 28 (FIG. 24A), and reversely rotates to slide the buffer sheets P1, P2, and P3 downward on the intermediate tray 29 to be brought into abutment against a stopper 31 (FIG. 24B) Subsequent sheets are sequentially stacked on the buffer sheets, and when the predetermined number of sheets are stacked, the sheet bundle is stapled by the stapler. After the series of operations are repeated, the stapled sheet bundles are sequentially stacked on the stack tray 28.
Thus, in the conventional sheet processing apparatus, even when sheets are supplied while the sheet bundle is stapled by the stapler, the sheets are buffered to the buffering unit, thereby preventing a flow of the supplied sheet from being inhibited.
However, the conventional sheet processing apparatus has the following two problems.
(First Problem)
In recent years, with high productivity of an image forming apparatus, an image forming processing speed in an apparatus main body is increased, with the result that a distance between sheets supplied from the apparatus main body to the sheet processing apparatus becomes smaller.
As a result, as shown in FIG. 25, in a process in which the buffer sheets P1, P2, and P3 are slid downward on an intermediate tray 29 and brought into abutment against the stopper 31, a first sheet P1 of the subsequent sheet bundle may be conveyed thereto. However, at this time, the rocking roller pair 27 does not open since the rocking roller pair brings the buffer sheets into abutment against the stopper 31. Thus, the rocking roller pair 27 cannot receive the first sheet P1 of the subsequent sheet bundle. Accordingly, the first sheet P1 of the subsequent sheet bundle may be brought into abutment against rocking roller pair 27 and become a jammed sheet.
Accordingly, it is considered that processing such as stapling processing of a stapler, delivering processing of the stapled sheet bundle, and trailing edge aligning processing of bringing the buffer sheet into abutment against the stopper 31 is sped up. However, as the processing is sped up, a drive source of the apparatus is increased in size, thereby arising another problem in that it is difficult to reduce the entire apparatus in size.
(Second Problem)
In a case where the number of sheets of the sheet bundle is less than the number of buffer sheets to be buffered to the buffering unit, when the preceding sheet bundle is being stapled by the stapler, a sheet of the subsequent sheet bundle is supplied to the buffering unit, and further the first few sheets of the subsequent sheet bundle are also supplied. For this reason, there is a problem in that, when the subsequent sheet bundle is to be stapled, sheets of a further subsequent sheet bundle are also stapled together.
For example, it is assumed that a maximum number of buffer sheets which can be buffered to the buffering unit is three. When a bundle of two sheets which are lower then the maximum buffer sheet number, the two sheets of the sheet bundle are buffered in a first job. However, since the maximum buffer sheet number is three, one more sheet can be buffered. As a result, a first sheet of another sheet bundle is buffered in the next job. When the buffered sheets are stapled in this state, there arises a problem in that the two sheets of the sheet bundle in the first job and the one sheet of the subsequent sheet bundle in the next job are stapled together.
The above-mentioned problems arise not only in a sheet processing unit serving as a staple unit for stapling a sheet bundle, but also in a sheet processing unit serving as a punch unit for punching a sheet bundle.
The image forming apparatus including the sheet processing apparatus having the above-mentioned problems in the apparatus main body has a problem in that high productivity in image formation is inhibited by the sheet processing apparatus.